1. Field of the Invention
This invention relates to coated abrasive products having an isocyanurate resin binder which holds and supports a coating of abrasive granules on a backing sheet. In another aspect, the invention relates to a method for making such a coated abrasive product employing a one-part isocyanatecatalyst system curable to isocyanurate resin wherein the catalyst is encapsulated.
2. Prior Art
Coated abrasives generally comprise a flexible backing upon which adhesive holds and supports a coating of abrasive granules. The backing may be paper, cloth, film, vulcanized fiber, etc. or a combination of one or more of these materials. The abrasive granules may be formed of flint, garnet, aluminum oxide, alumina:zirconia, diamond, silicon carbide, etc. Popular present day binders are phenolic resins, hide glue and varnish. Phenolic resins preferably include those of the phenol-aldehyde type. Besides phenolic resins, hide glue and varnish, other known resinous binder materials employed in the preparation of coated abrasive products include epoxy resins, ureaformaldehyde and polyurethane resins.
The coated abrasive may employ a "make" coat of resinous binder material which is utilized to secure the ends of the abrasive granules onto the sheet as the granules are oriented and a "size" coat of resinous binder material over the make coat which provides for firm adherent bonding of the abrasive granules to the sheet. The size coating resin may be the same as the make coating resin or of different resinous material.
The popular presently used binders for coated abrasives have many disadvantages. Hide glue make coatings are generally required to be applied hot (about 150.degree. F.) and must be cooled below their gel temperature before another coat can be applied. Hide glue size coatings are also generally applied hot, requiring drying times on the order of 30 minutes to 2 hours at 100.degree.-135.degree. F. Hide glue is also water-soluble and may result in removal of the abrasive granules upon exposure to moisture. Varnish make coatings generally require cure or drying times of about 8 hours at about 195.degree. F. (90.degree. C.). Phenolic size coatings require at least about 2 hours to cure at 195.degree. F. Phenolic make coatings are also usually pre-cured for at least one hour.
At first, polyisocyanurate resin may be thought to be a very likely candidate for the binder material of coated abrasive products, especially in view of the teaching of assignee's British Pat. No. 1,398,244. This patent discloses preparation of low-density abrasive products utilizing isocyanurate but not coated abrasives. This speculation has proven false because of the peculiar nature of the catalysts used to form this resin from the isocyanate starting material. The known catalysts for trimerizing isocyanates can be divided into two general classes. First, those which rapidly trimerize isocyanate exothermically at room temperature, and, second, those which trimerize isocyanates only at elevated temperatures and with prolonged reaction times. The first group of catalysts lacks latency and therefore their use is generally limited to two-part compositions with the parts mixed together very shortly before use. The second group of catalysts generally have a short shelf life, e.g., several weeks after mixing with the isocyanate starting materials, and requires long cure times, on the order of several hours at elevated temperatures.
An adhesive binder system which has a rapid cure (fast reaction rate) is desired in the production of coated abrasive products. Too rapid a cure would be incompatible with presently used production methods because it will cause advancement of the resin viscosity and perhaps result in failure to properly secure the abrasive granules to the substrate backing or foul coating equipment with prematurely cured adhesive. Both of these situations would be intolerable since isocyanurate is a thermoset material which is difficult to remove. Certain methods of metering have been developed to meter out stoichiometric quantities of isocyanate and catalyst. These methods, however, have generally been found to be unsuitable for the preparation of coated abrasive products because of application difficulties and resin waste.
The use of the second group of catalysts to prepare coated abrasive products is unacceptable because of the stringent cure conditions required which often result in unacceptable degradation of the backing. Additionally, the long cure times would make the process for producing such abrasive products uneconomical since prolonged time in the curing oven would be required with attendant increases in energy use and equipment costs.